Enrollment of healthy people like them in insurance under the Affordable Care Act is key to offsetting the costs of older, less healthy buyers.

Brad Stevens is 54 years old and not so invincible anymore. He has been uninsured for most of his adult life — "ever since about 24 when I finished college," he says. "Basically, I've always tried to take care of myself and be healthy and exercise and eat right and take vitamins and that type of thing."

During the three decades Stevens has spent without health coverage, there have been numerous attempts to curb the ranks of the uninsured in the U.S. Now, the Affordable Care Act is changing the nation's insurance market.

Stevens, who lives in Lakeport, Calif., has plenty of company. Twenty percent of California's population is uninsured, some 5 million people who could gain coverage under the health law.

Stevens wasn't interested in the debate over how best to provide health care to the uninsured. He didn't view it as an issue for him. "I'm the epitome of health, and so I didn't have much concern. My health care was working out every day, eating right and taking care of myself," he says.

But that began to change. One day eight years ago, while cycling on a country road in Lakeport, Stevens took a spill and separated his shoulder.

By then he had worked in a number of jobs, usually working with his hands. He'd been a building contractor, manager of a fruit warehouse and lately, a massage therapist. In pain for six months and with only $2,000 to his name, Stevens got medical help from the county.

That wasn't the end of his health troubles. One day, he says, "Boom! I ended up with cancer, thyroid cancer."

Because of the health care law, Stevens will qualify for Medicaid, the federal program for low-income families and individuals. Before the health law, being covered by Medicaid was available only to children, pregnant women and the disabled. Now all low-income people in states that are expanding Medicaid will qualify. Stevens earns less than $15,000 a year in his struggling massage business.

Stevens says he is relieved that he won't have to worry about being denied coverage for pre-existing conditions, another change made by the law.

Besides his separated shoulder and thyroid cancer, which is being controlled with medicine, there's a history of multiple sclerosis in his family. In the past, he avoided getting tested for the disease out of fear that the results would make it impossible to qualify for insurance.

Stevens says he worries about his massage patients. "People are hurting, and they need help," he says. "I don't think Congress has a clue. Fifty-five-year-old people are falling apart. They can't swing a hammer till they're 70 or 80, like some congressman who sits at a desk and jaws."

In the past, he occasionally went to the county fairgrounds when a volunteer group offered free medical checkups. Now he's anxious to sign up for Medicaid, although he hopes he won't have any more health problems.

"I hope I don't need to use it," he says. "I'm thinking I'm going to get ... peace of mind from it."

With the launch of the enrollment period for the Affordable Care Act this week, we're looking at how the law might change people's lives. Much of the attention so far has been focused on the health insurance exchanges, those online marketplaces, where people can shop for insurance policies. But the exchanges are also a place for people with low-income can find out if they qualify for the expanded Medicaid program.

Until now, Medicaid has been mostly limited to pregnant women, families with children and the disabled. In a majority of states that will continue to be the case. But in at least 22 states, and the District of Columbia, the eligibility for Medicaid has expanded. And as a result, single adults with low incomes can get Medicaid coverage for the first time. In California alone, some one million people will become eligible.

Reporter Sarah Varney met one California man who has not had health insurance for years. For him, the expansion is happening just in time.

SARAH VARNEY, BYLINE: Have you heard of the so-called Young Invincibles? Young people who think nothing bad can happen to them? Brad Stevens is a 54-year-old Young Invincible, recently reformed.

BRAD STEVENS: I have been uninsured for most of my, let's see, adult life. Ever since, boy, about 24 when I finished college. I've always tried to take care of myself and be healthy, exercise and eat right, and take vitamins and that type of thing and boom.

VARNEY: And then one day...

STEVENS: Boom, I ended up with cancer, thyroid cancer.

VARNEY: During the three decades Brad has spent without health coverage, there have been numerous attempts to curb the growing ranks of the uninsured in the U.S., by revamping the nation's insurance market.

Were you looking for a solution to come out of Washington?

STEVENS: No, I did not think it could to affect me at all. I'm the epitome of health, you know, and so I didn't have much concern. My health care was working out every day, eating right and taking care of myself.

VARNEY: And then, one day, while cycling on the country roads in Lakeport, a rural town on the western shore of Clear Lake where he lives, Brad took a spill and separated his shoulder. By then he'd had a number of jobs, usually working with his hands: building contractor, manager of a fruit warehouse, massage therapist. In pain for six months and with only $2,000 to his name, Brad got medical help from the county. Now he bikes at the gym where he sees his clients.

STEVENS: I don't need to take that risk anymore. Let me just get on a stationary bicycle...

(LAUGHTER)

STEVENS: ...and I'll bike on that for four hours and watch TV, because I don't have a TV. So...

(LAUGHTER)

STEVENS: ...I'll watch it at the club.

VARNEY: As Brad got older and started needing medical care, he started paying attention to the political debate over health care.

STEVENS: I really believe that in five, 10, 15, maybe even 30 or 40 years down the road, we will have universal health care. And this was like the first step.

VARNEY: Did you have any idea that you would qualify for Medicaid?

STEVENS: No. No. Uh-uh. No. Uh-uh.

VARNEY: Brad says he's relieved that he won't have to worry about being denied coverage for pre-existing conditions. Besides his battle with thyroid cancer - which is being controlled with medications - and a separated shoulder, his mother and aunt have multiple sclerosis. He's avoided getting tested out of fear the results would mar his insurance record. He earns less than $15,000 a year in his struggling massage business. Most of Brad's patients are disabled, busted backs and hips.

STEVENS: People are hurting, you know, and they need help. And I don't think Congress has a clue, frankly. 55-year-old people are falling apart. You know? Hey, they can't swing a hammer till they're 70 or 80, like some congressman who, you know, sits at desk and jaws.

VARNEY: Brad occasionally went to the county fairgrounds when a volunteer group offered free medical check-ups. Now, he's anxious to sign up for Medicaid.

So, what do you think you'll do with this insurance once you have it? do you think you'll actually use it? Or do you just think it'll give you kind of peace of mind?

STEVENS: I hope...

(LAUGHTER)

STEVENS: ...I don't need to use it. You know? But yeah, I'm thinking I'm going to get a peace of mind from it.

VARNEY: For NPR News, I'm Sarah Varney.

BLOCK: Both of the stories we just heard were part of a collaboration between NPR and Kaiser Health News.